The present invention generally relates to a metal insulator semiconductor field effect transistor, and more particularly to a vertical type metal insulator semiconductor field effect transistor having a high drivability, such as a power metal oxide semiconductor (MOS) field effect transistor (FET). Further, the present invention is concerned with a semiconductor device having such field effect transistors.
Recently, power MOSFETs have been used for controlling motors and switching power sources which need a large amount of current. Referring to FIG. 1, there is illustrated a conventional vertical type power MOSFET used for controlling a large amount of current. The power MOSFET in FIG. 1 has an n.sup.+ -type silicon substrate 1 and a silicon epitaxial layer 2 functioning as a drain region. A back gate region 5 having portions of the p.sup.+ type or p-type is formed in the epitaxial region 5. A source region 7 of the n.sup.+ -type (which has two portions in its cross section) is formed in the back gate region 5. A channel 8 is formed on a surface portion of the back gate region 5 located between a portion of the silicon epitaxial layer 2 under a gate oxide film 3 and a portion of the source region 7. The gate oxide film 3 is completely covered by an insulation film 10. The channel length is equal to a difference between the diffusion distances of the back gate region 5 and the source region 7 in the transversal direction. Hereafter, the above-mentioned structure is referred to as a cell 12, and a region in which a number of cells 12 are arranged close to each other is referred to as an operation area 13. The back gate region 5 is electrically connected to the source region 7 via a source electrode 11a, and is fixed to a potential of the source electrode 11a.
The source electrode 11a also functions as a field electrode. A polarity reverse preventing region 9 of the n.sup.+ -type for preventing the polarity of the epitaxial layer 2 from reversing is provided so that it surrounds a guard ring region 6. An equal potential electrode 11b is electrically connected to the polarity reverse preventing region 9.
In order to enhance the maximum driving voltage of the above power MOSFET, an arrangement shown in FIG. 2 is employed. The p.sup.+ -type guard ring region 6 is provided in peripheral portions of a semiconductor chip 14 so that it surrounds the operation area 13. As shown in FIG. 1, impurities of the guard ring region 6 are deeply diffused into the epitaxial layer 2. A peripheral portion 6a of the guard ring region 6 has a small curvature. The guard ring region 6 is electrically connected to the source region 7 by the source electrode 11a.
It is impossible to form the back gate region 5 deeply in the epitaxial layer 2 with respect to the channel length. Thus, a peripheral portion of a PN junction between the back gate region 5 and the epitaxial layer 2 has a large curvature. As a result, it is very difficult to obtain a large breakdown voltage. The guard ring region 6 is provided for the purpose of increasing the breakdown voltage of the power MOSFET shown in FIG. 1. When a reverse voltage is applied between the source region 7 and the drain region 2, the electric field of a depletion layer DP illustrated by the broken line in FIG. 1 extends from the PN junction around the guard ring region 6 to the operation area 13. This electric field functions to reduce the electric field extending from the PN junctions in the cells 12 to the guard ring region 6. Thereby, it becomes difficult for the avalanche breakdown to occur in the PN junctions of the cells 12, so that the breakdown voltage of the power MOSFET is increased.
As shown in FIG. 2, p.sup.+ -type regions 6a, 6b and 6c are formed under gate pads, source pads and electrodes for sending signals applied to gate pads to the gate electrodes 4 of the cells 12. These p.sup.+ -type regions 6a, 6b and 6c function to reduce parasitic capacitances between the epitaxial layer 2 and the above pads or electrodes, so that currents are caused to uniformly pass through an entire chip.
The conventional layer structure shown in FIG. 1 is capable of preventing the avalanche breakdown from occurring at a peripheral portion 5a of the PN junction between the guard ring region 6 and the epitaxial layer 2. However, there is a problem in which the avalanche breakdown is liable to take place at an internal portion 5b of the PN junction between the guard ring region 6 and the epitaxial layer 2. That is, it is difficult for the cells 12 located at the peripheral portion of the operation area 13 to be broken, and on the other hand, it is easy for the cells 12 located at an inner portion of the operation area 13 to be broken.
When the power MOSFETs, each having the structure shown in FIG. 1, are used for controlling the drive of a motor, a flyback voltage resulting from an inductance of a coil of the motor is applied to the cells 12, each cell having a small area. Then, almost all of flyback energy is consumed in the cells 12. Each of the cells 12 has a small breakdown voltage. In addition, during this operation, a parasitic transistor formed by the epitaxial layer 2, the back gate region 5 and the source region 7, is operating. Thus, a thermal runaway will occur in some of the power MOSFETs, and they will be easily damaged.
Conventionally, in order to eliminate the above problem, as shown in FIG. 3A, a clamp diode which has an avalanche breakdown voltage lower than that of the power MOSFET is externally connected. Alternatively, as shown in FIG. 3B, a series circuit of a capacitor C and a resistor R is externally connected between the source and drain of the power MOSFET. The clamp diode in FIG. 3A and the series circuit in FIG. 3B absorb the flyback energy and protect the power MOSFETs.
However, the structures shown in FIGS. 3A and 3B need an increased number of structural parts electrically connected to the power MOSFET and increase the production cost of application circuit devices. Further, a large sized chip will be needed to form the clamp diode or the series circuit if they are formed on the chip. This also increases the production cost.